Bone tissue consists of approximately 60-67% by weight of calcium phosphate crystals finely dispersed in a collagenous matrix, and also contains about 10% water. Some bone-forming reactions have been described. However, neither the actual sequence nor the specific mechanisms leading to bone formation are fully understood. It is logical, however, to consider bone formation as the result of two major trains of events, i.e., a first one that produces the collagen precursor matrix; the next a sequence of steps that leads to calcification, i.e., the mineralization of the organic matrix. These two phases are distinct, since it is possible to microscopically distinguish the calcified tissue from the non-calcified (osteoid) tissue in bone tissue that is being laid down.
Cementless fixation of permanent implants has become a widespread surgical procedure which aids in avoiding some of the late complications of cemented prosthesis. See, "Total joint replacement arthroplasty without cement", Galante, J. O., guest editor Clin. Orthop. 176, section I, symposium, pages 2-114 (1983); Morscher, E., "Cementless total hip arthroplasty", Clin. Orthop. 181, 76-91 (1983); Eftekhar, N. S., "Long term results of total hip arthroplasty", Clin. Orthop. 225, 207-217 (1987). In principle, cementless fixation can be achieved by using any of three methods: bone tissue ingrowth in porous coatings, bone tissue apposition on undulated, grooved, or surface structured prostheses, and fixation through chemical reaction with a bioactive implant surfaces. See, Hulbert, S. F., Young, F. A., Mathews, R. S., Klawitter, J. J., Talbert, C. D., Stelling, F. H., "Potential of ceramic materials as permanently implantable skeletal prostheses", J. Biomed. Mater. Res. 4, 433-456 (1970); Griss, P., Silber, R., Merkle, B., Haehner, K., Heimke, G., Krempien, B., "Biomechanically induced tissue reactions after Al.sub.2 O.sub.3 -ceramic hip joint replacement. Experimental study and early clinical results", J. Biomed. Mater. Res. Symp. No. 7, 519-528 (1976); Hench, L. L., Splinter, R. J., Allen, W. C., Greenlee, T. K., Jr., "Bonding mechanisms at the interface of ceramic prosthetic materials", J. Biomed. Mater. Res. Symp. 2, 117-141 (1973). Common to these three principles of fixation is the necessity that the surrounding tissues establish and maintain a bond with the device. This can be contrasted with the cemented reconstructions of which failure is invariably associated with destruction or resorption of surrounding bone tissue. See, Charnley, J., "Low-friction arthroplasty of the hip", Springer-Verlag, Berlin, Heidelberg, New York (1979); Ducheyne, P., "The fixation of permanent implants: a functional assessment", Function Behavior of Orthopaedic Biomaterials, Vol. II, CRC Press, Boca Raton, Fla. (1984).
Cementless fixation methods are not free from limitations. When porous coated devices are used the device is not permanently fixed at the time of surgery. A finite time is needed for bone tissue to develop in the porous coating interstices and eventually create sufficient fixation for patients to use their reconstructed joints fully.
It is known that bioactive materials such as calcium phosphate ceramics (CPC) provide direct bone contact at the implant-bone interface and guide bone formation along their surface. These effects are termed collectively osteoconduction. See, Gross, V., Schmitz, H. J., Strunz, V., "Surface Activities of bioactive glass, aluminum oxide, and titanium in a living environment. In: "Bioceramics: material characteristics versus in vivo behavior", Ed P. Ducheyne, J. Lemons, Ann. N.Y. Acad. Sci., 523 (1988); R. LeGeros et al., "Significance of the porosity and physical chemistry of calcium phosphate ceramics: biodegradation-bioresorption", In: "Bioceramics: material characteristics versus in vivo behavior", Ed P. Ducheyne, J. Lemons, Ann. N.Y. Acad. Sci., 523 (1988). This property of bioactive ceramics is attractive, not only because it may help in averting long term bone tissue resorption, but also because it enhances early bone tissue formation in porous metal coatings such that full weight bearing can be allowed much sooner after surgery. Calcium phosphate ceramics, although widely known to be bone conductive materials, do not, however, have the property of osteo-induction, since they do not promote bone tissue formation in non-osseous implantation sites.
The enhancement of bony ingrowth was first documented with slip cast coatings. Ducheyne, P., Hench, L. L., "Comparison of the skeletal fixation of porous and bioreactive materials", Trans. 1st Mtg. Europ. Soc. Biomater, p. 2PS, September, 1977, Strasbourg; Ducheyne, P., Hench, L. L., Kagan, A., Martens, M., Mulier, J. C., "The effect of hydroxyapatite impregnation on bonding of porous coated implants", Trans. 5th annual mtg., Soc. Biomat. p. 30 (1979); Ducheyne, P., Hench, L. L., Kagan, A., Martens, M., Burssens, A., Mulier, J. C., "The effect of hydroxyapatite impregnation on skeletal bonding of porous coated implants", J. Biomed. Mater. Res. 14, 225-237 (1980). A porous stainless steel fiber network was coated with a slip cast CPC lining, and a marked increase of bone ingrowth was observed in comparison to the same porous metal without the CPC lining. This effect was pronounced at 2 and 4 weeks, but had disappeared at 12 weeks, because the slower full ingrowth without CPC lining had achieved the same level of ingrowth as that of the earlier extensive ingrowth caused by the osteoconductive lining.
Subsequently, the effect was studied mostly with plasma sprayed coatings, by numerous researchers. The studies to date, with the exception of one, have confirmed the beneficial effect of calcium phosphate based ceramic linings. See, J. L. Berry, J. M. Geiger, J. M. Moran, J. S. Skraba, A. S. Greenwald, "Use of tricalcium phosphate or electrical stimulation to enhance the bone-porous implant interface", J. Biomed. Mater.Res. 20, 65-77 (1986); H. C. Eschenroeder, R. E. McLaughlin, S. I. Reger, "Enhanced stabilization of porous coated metal implants with tricalcium phosphate granules", Clin. Orthop. 216, 234-246 (1987); D. P. Rivero, J. Fox, A. K. Skipor, R. M. Urban, J. O. Galante, "Calcium phosphate-coated porous titanium implants for enhanced skeletal fixation", J. Biomed. Mater. Res. 22, 191-202 (1988); M. D. Mayor, J. B. Collier, C. K. Hanes, "Enhanced early fixation of porous coated implants using tricalcium phosphate", Trans. 32nd ORS, 348 (1986); Cook, S. D., Thomas, K. A., Kay, J. F., Jarcho, M., "Hydroxyapatite-Coated porous titanium for use as an orthopaedic biologic attachment system" , Clin. Orthop. 230, 303-312 (1988); H. Oonishi, T. Sugimoto, H. Ishimaru, E. Tsuji, S. Kushitani, T., Nasbashima, M. Aona, K. Maeda, N. Murata, "Comparison of bone ingrowth into Ti-6A1-4V beads coated and uncoated with hydroxyapatite", Trans. 3rd World Biomat. Conf., Kyoto, p. 584 (1988). Yet, the magnitude of the effect has varied from study to study, and was not as pronounced as in an experiment performed by the present inventor. See, Ducheyne, et al., "The effect of hydroxyapatite impregnation . . . ", supra. More recently, it has been found that porous titanium, spherical bead coatings, plasma sprayed with two calcium phosphate powders (either hydroxyapatite or beta-tricalcium phosphate before spraying) also did not yield a clinically meaningful effect. See, Ducheyne, P., Radin, S., Cuckler, J. M., "Bioactive ceramic coatings on metal: structure property relationships of surfaces and interfaces", "Bioceramics 1988" Ed. H. Oonishi, Ishiyaku Euroamerica, Tokyo (1988 in press).
The variability of the effect among the studies noted suggests materials and processing induced parametric influences. The extensive characterization of some plasma sprayed coatings has unveiled that considerable changes of the physical and chemical characteristics of the ceramic subsequent to the deposition are possible. Specifically, differences in chemical composition, the trace ions present, the phases and their crystal structure, macro- and micro-porosity in the ceramic film, specific surface area, thickness, size and morphology of the pores and of the porous coating itself, and the chemical characteristics of the underlying metal may have occurred among the various studies.
Much of the prior art teaches the use of plasma spray techniques to form ceramic coating. Limitations of plasma spray coatings include: possible clogging of the surface porosity, thereby obstructing bone tissue ingrowth; difficulty in producing a uniform coating--although the HA can flow at the time of impact, plasma spraying is still very much a line of sight process, thus, it is not possible to coat all surfaces evenly, and certainly not the deeper layers of the coating, or the substrate; and finally if viscous flow is wanted, high temperatures are reached by the powders and uncontrolled, and thus unwanted transformation reactions can occur. Efforts to avoid these transformation reactions can be successful by minimizing the time of flight. However, a low intensity of viscous flow will result from this and thus, incomplete coverage of the metal can be the result. Thus, at present, it is difficult to obtain an optimal end-product, however, improved ceramic powders may overcome these limitations and provide useful coatings using plasma spraying techniques.
The search thus continues for the optimal characteristics of the ceramic and for a process by which calcium phosphate ceramics may be deposited upon porous metal surfaces in a uniform manner and with predictable results. Although it is possible to coat flat plates of metals such as titanium by electrophoretic deposition, actual experiments by the applicants to deposit a uniform ceramic film on porous titanium using the information available prior to the current invention were unsuccessful. Referring to FIG. 1 and FIG. 2, there is illustrated a portion of a porous metallic device which is comprised of a woven mesh. Analysis of porous metallic devices with failed ceramic coatings showed that the ceramic particles were deposited primarily in the few areas indicated in FIG. 1 and FIG. 2. It is apparent that only those areas that were well exposed to the flow of particles were covered. During electrophoresis, the particles are electrically attracted to the metal; with a finite amount of particles in the solution, the particles will migrate first to the most accessible areas 20 of the metallic device 10, such as the tops of the wires. Further, some particles will be able to cross the potential field created by the wires if the particles are sufficiently far from each of the wires and be deposited on the substrate 30 to which the woven mesh is affixed. However, very few particles actually overcome this combination of attractive forces and adhere to the interstitial areas 15. Thus, the known electrophoretic process to coat flat titanium plates does not provide an adequately coated device.
Therefore, it can be seen that there remains a long felt, yet unfulfilled need for both a material with optimum characteristics and a deposition process which will allow uniform deposition of ceramic materials in a repeatable and commercially viable manner.